GSX is both a 32 bit and a 64 bit program, at least the parts of it that interface with the simulator at the lowest level.
This means, you can use it today on FSX, and it will work as a 32 bit program, but if you will eventually switch to a 64 bit simulator (P3D4, which is what we suggest to use), the program will still work with it, since it also compatible with 64 bit.